Method for hardening cast iron



Patented Jan. 16, 1951 METHOD FOR HARDENIN G CAST IRON George B. Farnsworth, Ojai, Calif., assignor of one-fourth to Thomas E. Piper, Santa Monica,

Calif.

No Drawing. Application September 20, 1947, Serial No. 775,373

2 Claims.

The present invention relates to a method for hardening cast iron.

In the past many attempts have been made to harden cast iron but so far as applicant is aware no one has ever succeeded in hardening more than a very thin shell on an iron casting and such mold is, of course, well known, but since such chilled portions are very difiicult to machine, this practice has been limited to uses where a minimum of finish machining of the chill hardened portions was required.

An object of the present invention is to harden cast iron.

Another object is to develop a new and simple process for hardening cast iron.

Another object is to make an improved hea treating process for cast iron.

In practicing my invention, any type of gray iron casting may be employed, and the casting may be hardened either in the malleable or ordinary; condition. Since a slight modification is required in the hardening of malleable castings, I first shall explain how the process is performed on-ordinary iron castings.

First, if any machining is to be done on the castings, this is done before the hardening process is initiated, since once hardened, the castings are too hard for ordinary machining, except where finish grinding of some parts may be required. Such finish grinding can of course be done even on hardened work. Once the initial heating which begins the present hardening process is understaken, the castings must never be permitted to cool below a temperature of approximately 450 vF. until the process is completed. In the case of ordinary castings, no preannealing should be attempted prior to the beginning of the hardening process, since such should be sufficiently slow so as not to exert expansion stresses on the castings, but to permit gradual and uniform expansion of all the parts. In the case of castings .of extremely critical shapes, for example a pulley having large hub, heavy rim, and relatively thin spokes, unless the parts are brought up to temperature very gradually, fracture of the spokes is almost certain to occur. The castings are held at the 1700 F. temperature for a time, dependi ng on the size and sections of the castings involved. For example, a casting having an average maximum two-inch section would be held at this temperature for approximately one hour.

After soaking at the 1700 temperature for the required length of time, which opens the grain structure of the cast iron, the castings are allowed to cool to a temperature'between 450 F. and 700 F. This cooling must be permitted to occur slowly, either in a furnace, or if the furnace capacity is desired for other uses, the castings may be packed for example in air slaked lime.

When the parts thus have cooled gradually to the lower temperature which, as stated, must not be less than 450 F., the castings are carburized.

annealing has a tendency to cause cracks in main a suitable liquid carburizer.

The carburizing may be performed in the same manner as with steel, for example, by packing in a suitable carburizing agent or by immersing Since these carburizing agents are well-known to the art, it is not necessary to specify them in detail.

In the case where solid or granular carburizing material is employed, after the casting is packed in the manner usually employed for hardening steel, the casting in its investment of carburizing material, is gradually broughtup to a temperature of 1700 F. and is maintained at the carburizing temperature for a time determined in each instance by the depth to which the carburizing is desired to penetrate. After the carburizing period has elapsed, the casting again is allowed to cool slowly to a temperature slightly above 450 F. The casting then is removed from the carburizer and the dust from the carburizing pack is removed with an air blast or brush.

Still maintaining the casting at a temperature not below 450 F. it again is placed in the furnace and is reheated to a temperature of approximately 1450 to 1500 F. The higher of these two temperatures is better. The casting then is quenched in a good grade of quenching oil and then is drawn. The temperature of the draw is governed by the finished hardness desired. Usually a drawing temperature of 450 F. to 500 F. covers the range desired.

In using a liquid carburizing agent, the prom ess is generally similar to that described for the solid carburizing agent with one or two exceptions. These exceptions are as follows: After machining, and the first heating to 1700 F. and the gradual cooling to 450 F. to 700 F., and before immersing the casting in the carburizing bath, the casting first is slowly re-heated to a temperature between 1250 F. to 1400 F. It then is immersed in the liquid carburizing bath which is maintained at a temperature between 1650 F. and 1700 F. The casting is permitted to remain in the carburizr in the above-mentioned temperature range for a length of time depending upon the cross-section of the casting and the depth of penetration of carburization' desired. Usually a period of about two hours will sufiice for ordinary depth of cast. After the carburizing is completed the casting is quenched in liq-' uid salts at a temperature of 425 F. and then is allowed. to cool in the open air.

No drawing is required in the case of castings quenched in the liquid salt bath, but it should be borne in mind that the casting must be permitted to cool from the temperature of the quench to room temperature before attempting to wash off the coating of salt which solidifies and adheres to the casting when it is withdrawn from the salt bath. Otherwise the castings are very the grain structure in the casting, such time be ing longer for castings of greater mass and being of the order of one hour for a casting having maximum thickness of two inches, cooling the casting slowly to a temperature between 450 F. and 700 E, heating the casting; while at least partially enclosed in a carburizing material to a temperature of 1700 F., maintaining said cast ing while so enclosed at said latter temperature for a length of time determined by the depth to which it is desired to harden the casting, slowly cooling the casting to a temperature slightly over 450 F., removing, the casting from the carburizer, heating the casting to a temperature between 1450,F. and1500 F., quenching the casting in oil, and drawing the casting at temperature apapt to fracture if not permitted to cool gradually from the salt quenching bath temperature to room temperature.

In hardening malleable castings after the malleablizing process has been completed, the castings are'machined. After machining the hardening process is carried out the same as with ordinary castings, except that in all cases where temperatures above 1600 F. are specified, these temperatures must be limited to 1600 F. maximum. Any temperature above 1600 F. will deinvention. It is; desired, therefore, not to limit theinvention except as set forth in the following claims.

Iclaim:

1. The method of hardening a non-malleable gray iron casting which comprises heating the casting to atemperature of 1700F. at a rate slow enough to minimize expansion stresses due to heating, maintaining the casting at said temperature for a length of time sufficient to open proximating 400 F. to 500 F.

2. The method of hardening a non-malleable gray iron casting which comprises heating the casting to a temperature of approximately 1700 F. at a rate slow enough to minimize expansion stresses due to heating, maintaining the casting at said temperature for a length of timevarying with the thickness of the casting and being of the order of one hour for a casting having maximum thickness of two inches, cooling the casting slowly to a temperature between 450 F. and 700 F., heating the casting in a carburizing material to a temperature of approximately 1650 F. to 1700 F., maintaining said casting in said carburizer at said latter temperature range for a length of time determined by the depth to which it is desired to harden the casting, slowly cooling the casting to atemperature slightly over 450 F., removing the casting from the carburizer, heating the casting to a temperature between 1450 F. and 1500 F., and quenching the castmg.

GEORGE E. FARNSWORTH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES, PATENTS Number Name Date 1,688,438 Schwartz Oct. 23, 1928 1,760,239 Lauenstein et al. May 27, 1930 1,894,752 Bozard et al Jan. 17, 1933 1,934,313 Lauenstein Nov. 7, 1933 1,980,578 Ford Nov. 13, 1934 2,008,452 Jewett et a1 July 16, 1935 FOREIGN PATENTS Number Country Date 418,873 Great Britain Nov. 1, 1934 OTHER REFERENCES Metals and Alloys, September 1931, page 131. 

1. THE METHOD OF HARDENING A NON-MALLEABLE GRAY IRON CASTING WHICH COMPRISES HEATING THE CASTING TO A TEMPERATURE OF 1700* F. AT A RATE SLOW ENOUGH TO MINIMIZE EXPANSION STRESSES DUE TO HEATING, MAINTAINING THE CASTING AT SAID TEMPERATURE FOR A LENGTH OF TIME SUFFICIENT TO OPEN THE GRAIN STRUCTURE IN THE CASTING, SUCH TIME BEING LONGER FOR CASTING OF GREATER MASS AND BEING OF THE ORDER OF ONE HOUR FOR A CASTING HAVING MAXIMUM THICKNESS OF TWO INCHES, COOLING THE CASTING SLOWLY TO A TEMPERATURE BETWEEN 450* F. AND 700* F., HEATING THE CASTING WHILE AT LEAST PARTIALLY ENCLOSED IN A CARBURIZING MATERIAL TO A TEMPERATURE OF 1700* F., MAINTAINING SAID CASTING WHILE SO ENCLOSED AT SAID LATTER TEMPERATURE FOR A LENGTH OF TIME DETERMINED BY THE DEPTH TO WHICH IT IS DESIRED TO HARDEN THE CASTING, SLOWLY COOLING THE CASTING TO A TEMPERATURE SLIGHTLY OVER 450* F., REMOVING THE CASTING FROM THE CARBURIZER, HEATING THE CASTING TO A TEMPERATURE BETWEEN 1450* F. AND 1500* F., QUENCHING THE CASTING IN OIL, AND DRAWING THE CASTING AT TEMPERATURE APPROXIMATING 400* F. TO 500* F. 